Problem: It takes 20 minutes for 9 people to paint 9 walls. How many minutes does it take 13 people to paint 13 walls?
Imagine that each person is assigned one wall, and all 9 people begin painting at the same time. Since everyone will finish painting their assigned wall after 20 minutes, it takes one person 20 minutes to paint one wall. If we have 13 people and 13 walls, we can again assign one wall to each person. Everyone will take 20 minutes to paint their assigned wall. In other words, it takes 20 minutes for 13 people to paint 13 walls.